1. Field of the Invention
The present invention relates generally to temperature probes, or sensor tips, of the type used for the control and safety monitoring of gaseous fuel burners as used in various heating, cooling and cooking appliances. In particular, the present invention relates to flame ionization sensor probes used in gas combustion control/safety environments where contamination coating of the probe shortens the useful life of the sensor.
2. Discussion of the Related Art
Flame ionization sensing provides known methods and apparatus for monitoring the presents of a flame for a gaseous fuel burner.
It is known that hydrocarbon gas flames conduct electricity because charged species (ions) are formed by the chemical reaction of the fuel and air. When an electrical potential is established across the flame, the ions form a conductive path, and a current flows. Using known components, the current flows through a circuit including a flame ionization sensor, a flame and a ground surface (flameholder or ground rod).
FIG. 1 illustrates a flame ionization sensor system 10 with a typical sensor/burner circuit loop as may be used in accordance with the present invention. Flame ionization sensor 11 having a probe 12, will be mounted near the burner 13. The output 15 of sensor 11 will be fed into a computer-controller 17. The sensor loop can provide information regarding the status of a flame 18 in the burner 13. If there is no flame, then the sensor 11 will not generate a signal which will cause the controller 17 to instruct the system to shut off fuel flow.
In utilizing a flame sensor as previously described, a voltage, such as a 120 AC voltage 21, will be applied across the sensor loop, with the flame holder, or burner 13, serving as the ground electrode 20. Flame contact between the sensor probe 12 and the burner 13 will close the circuit. The alternating current (AC) output of the sensor/ground circuit, can be rectified, if the ground electrode (flameholder or burner 13) is substantially larger in size than the positive electrode, since, due to the difference in electrode size, more current flows in one direction than in the other.
Flame ionization sensor probes 12 are thus electrodes, made out of a conductive material which is capable of withstanding high temperatures and steep temperature gradients. Hydrocarbon flames conduct electricity because of the charged species (ions) which are formed in the flame. Placing a voltage across the probe and the flameholder causes a current to flow when the flame closes the circuit.
Unfortunately it has been found that contaminants in the air stream of the fuel/air mixture can result in the build up of an insulating contamination layer on the probe, rendering it much less effective. At a certain level of coating, which prevents electron flow to the probe surface, the sensor is rendered useless, creating a premature or false system failure. Often these airborne contaminants are organosilicones found in personal and home care products which are oxidized by the flame 18 to silicon oxides (SiOx) which in turn build up through impact on the probe 12 providing the insulative contaminant coating.
It is thus desirable to find ways to increase the useful life of flame ionization sensor probes in spite of this insulative build up resulting from normal use of the flame ionization sensor system.
According to one embodiment of the present invention, the fact that the sensor tip, or probe, is exposed to the flame is taken advantage of and the probe is constructed and arranged according to materials and shapes which promote mechanical deformation of the sensor tip due to thermal expansion and contraction. Sufficient mechanical deformation will cause cracks to open in the contaminant layer surrounding the probe, breaking the insulative effect and allowing ions from the flame through to the probe thereby enabling the sensor to perform as intended even though insulative contaminate build up is present. The mechanical deformation may be sufficient to allow the probe to shed contaminant build up. The material of the probe will thus be selected to have a coefficient of thermal expansion (CTE) over the operating temperature range of the probe sufficient to allow such cracking or shedding of the contaminants to occur. Bimetal construction of the probe is a contemplated embodiment. Specially shaped probes such as helical, or corrugated shapes may be utilized in conjunction with material selection to further aid in contaminant layer cracking or shedding. Finally, some gain in contaminant build up prevention may also be had by specially shaping the probes to minimize SiOx particle impact on the probe.